Pywikibot runs a relatively large set of post-merge tests via GitHub Actions. Some of these tests use Beta Cluster wikis as their target for end-to-end testing of various features especially upcoming changes of new MW versions.
GitHub Actions uses Microsoft Azure to host it's runners. There are over 5000 (!) IP ranges listed at https://api.github.com/meta that GitHub Actions might make requests from. Due to unwanted crawlers, some of the IP addresses used by GitHub Actions have blocked and lead to http response 403 (see T399415).
So solve this problem, a proxy can be used. It’s usage is already available through requests package:
https://www.mediawiki.org/wiki/Manual:Pywikibot/Installation
https://requests.readthedocs.io/en/latest/user/advanced/#proxies
T399485#11011491
This might also increase the WM rate limits